Summary A simple method is proposed for predicting downhole shaped-charge gun performance based on the use of API RP 43, Edition 5, Sec. 1 data. API Sec. 1 has been the preferred method for assessing perforating gun system performance because of the simplicity of the test and its use of standard field guns fired at maximum shot densities and positioned as they would be in an actual well. The validity of the proposed method is demonstrated, allaying past concerns regarding the translation of data from Sec. 1 nonrock, nonstressed concrete targets to downhole conditions. The new method is based on an observed linear relationship between Edition 5, Sec. 1 and Sec. 2 penetration information. The applicability of the well-known Thompson relationship between formation compressive strength and perforator penetration to Edition 5, Sec. 2 and therefore to Sec. 1 data is shown. Incorporating necessary corrections for casing entrance hole size, downhole effective formation stress, and casing configurations different from those in the API test completes the translation of surface data to downhole conditions. Introduction Well flow performance is significantly affected by the extent of perforated hole penetration into the formation and the hole size in the casing, together with other fixed geometric parameters, such as shot density and gun phasing. Values of perforation penetration and hole size commonly available to the completion designer are provided by API RP 43, Edition 5 published data. These data are derived from tests at the surface and provide only limited simulation of subsurface conditions regarding formation physical properties and stress. Surface data can significantly vary from that to be expected downhole and must be converted to in-situ values before proceeding with well flow performance calculations. This conversion of RP 43, Edition 5 surface test performance to downhole involves consideration of the specific downhole formation physical properties, formation in-situ stress, casing properties, and the specific gun-to-casing configuration. This paper reviews the factors affecting downhole penetration and casing entrance hole size for perforating guns, discusses API data as a basis for predicting downhole performance, reviews API test results and results of tests performed specifically for this paper, and proposes a procedure for translating surface API data to downhole conditions. Factors Affecting Downhole Performance of Perforating Guns Gun-to-Casing Clearance. Clearance, the distance between the gun OD and the casing ID along the axis of the shaped-charge jet, can have a significant effect on total penetration, L, and casing entrance hole size, deh. As Fig. 1 shows, the estimated downhole L/deh of a commercial 33/8-in. gun perforating 7-in. casing varies from 14.45 in./0.35 in. to 7.65 in./0.21 in. when operated in the common eccentric running position (Fig. 1a). Values are constant when clearance is controlled (Figs. 1b and 1c), typical methods for positioning guns. The importance of the gun-to-casing arrangement is evident; it is the estimated downhole L/deh used in mathematical models to calculate well flow. Formation Strength. Compressive strength of the formation being penetrated influences perforation penetration depth. Thompson disclosed a semilog relationship between formation wet compressive strength and total penetration depth (casing thickness + cement thickness + formation penetration) for API Edition 4 Section 2 type targets. Penetration performance in unstressed sandstones and limestones of different compressive strengths is available in the literature. The high-strength end of Thompson's relationship (beyond 14,000 psi)was modified by data from Weeks' formula, resulting in the composite representation in Fig. 2. As Fig. 2 indicates, a perforating gun that provides a penetration of 11.8 in. in rock with a wet compressive strength of 7,000 psi (Point A) will penetrate less than 7 in. in a l4,000-psi formation (Point B). On the other hand, it would penetrate 15 in. in a 3,000-psi material (Point C). Use of mean wet uniaxial compressive strength, S, values is suggested in applying the above relationship. S is defined as the average of compressive strength values taken perpendicular and parallel to the bedding plane of saturated rock. It is related to the commonly used dry compressive strength measured perpendicular to the bedding plane, Sd, as follows: (1) When the value of S is unavailable, it may be approximated using formation porosity by means of Fig. 3, which is derived from the results of tests in several sandstones and limestones. These tests were performed in cores taken from surface outcrops, and results might be somewhat different in downhole formations. Data are limited below about 15% porosity for sandstones. Additional work should be done to improve the definition of the porosity/compressive strength relationship over a broader range of porosity and over a larger number of formation rocks. Nevertheless, in the range of 18% to 23% porosity, substantial and consistent data are available, providing a good curve fit. Formation Effective Stress. Formation effective stress is the overburden stress, po, minus the reservoir or pore pressure, pp: (2) where all factors are measured in psi. Stress reduces penetration (Fig. 4). Conceptually, increasing stress makes the formation appear stronger. When predicting downhole penetration performance from API test results, the effect can result in either a reduction or a gain in estimated penetration. The magnitude and nature of the effect will depend on stress in the formation compared with the stress in the RP 43, Edition 5 tests. Specifics are developed later. Hydrostatic Pressure. Although wellbore pressure tends to reduce penetration, the correlation for these hydrostatic pressure effects is included in the formation effective stress correction described above. Casing Strength. Casing grade affects perforation entrance hole diameter, deh, to a significant degree but exerts only a negligible effect on penetration across the typical API Sec. 1 test range of single casing-wall thicknesses. In single-casing completions, deh varies with the midrange Brinell hardness, H, of the particular casing grade, according to the following expression: (3) P. 171^
Flow rates through gun perforations calculated for radial-flow conditions and confirmed in laboratory tests indicated perforation efficiencies substantially lower than those observed in API RP 43 tests with linear-flow test targets. Observed perforation efficiencies were also strongly influenced by differential pressure: below, the API RP 43 standard of 200 psi, efficiencies were significantly decreased. Introduction From the inception of the gun perforating technique in 1932, the ultimate test of perforator effectiveness has been well productivity. As a result, much attention has been devoted to laboratory testing of perforators as a means of predicting and improving well perforators as a means of predicting and improving well performance. Laboratory procedures have evolved performance. Laboratory procedures have evolved over the years from simple single-shot penetration tests in steel to multishot tests in large cement targets using actual field guns. Shots at atmospheric pressure have been supplemented with tests under pressure and temperature environments simulative of down-hole conditions. Interest in laboratory flow properties of perforations entered the picture in 1953 with the perforations entered the picture in 1953 with the introduction of the laboratory flow test. This test, refined in 1956, culminated in the standard API RP 43 procedure in 1962. The API procedure until recently procedure in 1962. The API procedure until recently used Well Flow Index (WFI) as a means of comparing flow performance of perforations in the linear flow system employed. However, no true indication of the productive capacity of a perforation in the more nearly radial flow system that is encountered down hole could be derived from the WFI measurement. In an effort to provide more meaningful data, the API procedure was revised in 1971 to introduce Core Flow Efficiency (CFE) as the indicator of laboratory performance in the linear target. CFE is the ratio of flow from an actual perforation to flow from an ideal perforation of the same diameter and depth in the same target. While CFE represents a better basis than WFI for comparing perforation performance in the laboratory, the linear nature of flow in the API target still has raised questions as to the validity of applying CFE to down-hole conditions. Consequently, studies were undertaken to better define the liquid-flow characteristics, and particularly the flow efficiencies, of perforations under conditions more simulative of those down hole. Calculated flow and pressure distributions surrounding single perforations in linear laboratory targets were compared perforations in linear laboratory targets were compared with those existing around single perforations in a simplified down-hole model. Perforation-flow efficiencies for the down-hole model were calculated and confirmed in simulative experimental tests. Pressure and Flow Distributions - Ideal Perforations Pressure and Flow Distributions - Ideal Perforations Mathematical models of the linear target and a simplified down-hole system were developed to facilitate investigation of the flow and pressure distributions in the two systems. Initial work was done on ideal perforations since their flow rates are the basis for perforations since their flow rates are the basis for calculating perforation-flow efficiencies. The mathematical approach employed in analyzing the linear target is commonly referred to as the finite-difference technique. The target model is divided into a series of concentric segments as shown in Fig. 1. JPT P. 1095
A major objective of any well completion is to attain maximum production. The perforating equipment and production. The perforating equipment and techniques that are used have a very important bearing on determining the production that results. production that results. According to the nature of the reservoir, wells may be completed either naturally, with sand-control measures, or with formation stimulation by acidization and/or hydraulic fracturing. The wellbore-to-formation pressure relationship at the time of perforating may be overbalanced, balanced, or underbalanced. Perforating guns may be retrievable, Perforating guns may be retrievable, semi-expendable, or expendable; designed for operation through tubing or in open casing; run on wireline or on tubing. This paper describes the various combinations of guns and techniques that are in common use, with the advantages and disadvantages of each. The four basic performance parameters, i.e., shot performance parameters, i.e., shot density, perforation diameter, penetration depth, and gun phasing, are ranked in order of relative importance for natural, sand-control, and stimulated completions. Where justified, the author makes recommendations and draws conclusions. Introduction Perforating techniques to get best well productivity depend on the type of well completion; i.e., natural flow, sand control, or hydraulically fractured. Even within a particular completion method, choices of technique and equipment are constrained by the well configuration, wellbore fluid type, pressure, formation characteristics, and damage conditions. In general, the objective is to perforate in a way that produces minimum perforate in a way that produces minimum resistance to flow at the reservoir/perforated-system interface. This can be done by:–establishing well conditions that enhance cleanup of the perforations, and–choosing perforators and techniques for best flow performance. The intent of this paper is to describe the nature of the choices to be made, to discuss some of the factors that bear on these choices, and where justified, to make recommendations and draw conclusions as to the appropriate action. NATURAL COMPLETIONS The natural completion involves formations that do not require artificial alteration to permit worthwhile hydrocarbon production. This definition thus excludes completions requiring stimulation by fracturing or massive acidization, as well as those requiring gravel-packing or sand-consolidation treatment. Not excluded, however, are wells that are lightly treated with "mud acid" to cope with wellbore damage. Ideally, the well is perforated and placed directly on production. placed directly on production.
Improvements in wireline formation testing have been incorporated into a tool with multiple-set capabilities. The toot permits pretesting of the formation for permeable regions and checking of packer seal integrity, before sampling. Two fluid samples can be obtained on each trip and any number of pressure recordings can be taken during the same trip. Introduction The wireline formation-tester (FT) technique was introduced to provide confirmation of formation-fluid type, indications of productivity, and formation pressures. Various improvements have been made in the pressures. Various improvements have been made in the technique and interpretation methods have been developed for best use of the information from the recovered fluid samples and the pressure recordings. While the technique has been successful locally, it has not reached its full potential, basically because of the long rig time required with existing testers for multiple-zone testing. Once the tester was set in the well, it could not be repositioned at another level in the zone of interest. Consequently, any test failure caused by a tool setting in an impervious streak or by a packer-seal failure resulted in an extra trip in the well. packer-seal failure resulted in an extra trip in the well.Performance in many unconsolidated sands was not acceptable with these older tools. Techniques to combat the flow of sand into the tester were never completely successful; this sand flow caused undermining of the packer seal with subsequent mud-sample recovery. packer seal with subsequent mud-sample recovery. These factors combined to produce an over-all success ratio of about 70 percent for all formations and about 35 percent for unconsolidated sands. percent for unconsolidated sands.Also, toot redressing required between runs was extensive. This added to the over-all operating time unless additional tools were available at the well. Another limitation of existing FT tools was the insufficient accuracy of the recorded pressures (in the range of 2 to 3 percent). This, combined with the single-test-per-trip capability, often discouraged the use of these tools for recording several pressure measurements in a well. In summary, major limitations of these older tools were their inability to be repositioned, their single-test capacity, and the lack of a reliable means for testing the integrity of the seal before attempting a sample. To overcome these limitations, a new formation tester has been developed. Principal Features of the New Tester Principal Features of the New Tester The new tester has several distinguishing features as compared with the older tools. Several successive tool settings are possible without bringing the equipment out of the hole. Combined with this is a "pretest" capability that permits the operating engineer to ascertain, before attempting to take a sample, whether the packer is sealing properly and, if so, whether fluid flow is adequate to properly and, if so, whether fluid flow is adequate to obtain a sample in a reasonable period of time. Thus, if the tool is set and the packer seal fails, or if the indications are that the tool - is set in an impervious streak, the tool is simply retracted and moved to another position in the formation. If both seal and flow indications during pretest are satisfactory, a sample is taken. Two separate sample chambers make it possible to obtain two samples on a single trip into the well. JPT P. 1331
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